The pH value of blood is a commonly monitored metabolic parameter and provides a means for determining whether a proper physiological acid-base balance exists in the tested individual. A very delicate blood pH balance is present in humans. Normal values for arterial blood range from 7.25-7.45 with the lower and upper limits at which an individual can survive being 7.00 and 7.70, respectively. When a cardiac or medical arrest occurs, metabolic functions become anaerobic thus resulting in the production of excess acids. This causes blood pH to lower.
As medical practitioners are aware, accurate and rapid determination of whole blood pH promotes the safe and effective resuscitation and treatment of these arrest victims. Unfortunately, of the 640,000 cardiac arrests annually, 360,000 die before reaching the hospital partially due to improper blood pH levels which are incapable of being monitored on-site due to the prohibitory size of the current pH analyzers. Moreover, current analyzers are complex pieces of machinery requiring operation by skilled laboratory technicians. For example, calibration of these systems does not occur automatically. An electrolyte must be introduced into the measuring cassette by some form of mechanical manipulation before calibration can occur.
Additionally, the determination of pH on these systems is based on the glass electrode, as described by Cremer in 1906. These electrodes are composed of two half cells, one which generates a reference potential and the other being constructed of a glass membrane. These electrodes suffer from several shortcomings including the need for constant calibration as a result of half cell current drift, degradation of the membrane surface by cleaning solutions used to remove accumulated protein deposits, and the requirement of a large sample size.
U.S. Pat. No. 4,340,457 to Kater, discloses the use of metal electrodes to be used for in vivo potassium determination which can be stored wet in electrochemical contact. These electrodes, however, require further manipulation and hydration before calibration can occur. Further, the calibration liquid must be removed before use in taking the actual measurement.
U.S. Pat. Nos. 3,742,594 to Kleinberg, 3,926,766 to Niedrach et al and 4,561,963 to Owen et al, disclose the use of metal electrodes for electrochemical determination of various bodily fluids. All of these systems utilize wire probes, as opposed to planar structures which are not conducive to the use of chemical pastes as thin films such as those used in the present invention. Further, these electrodes require liquid tight seals making their manufacture very difficult.
U.S. Pat. No. 4,545,382 to Higgins et al discloses a metal electrode coated with a film of glucose oxidase. This system, however, utilizes a single sensor electrode, and is incapable of effecting a pH measurement.
U.S. Pat. Nos. 4,272,245 and 4,342,964 to Diamond et al disclose an apparatus and method for measuring the pH value of a blood sample which utilizes metal electrodes. This method, however, requires the presence of a liquid electrolyte used to calibrate the system and to provide an electrolytic bridge between the sample and the reference electrode within a cassette. Calibration does not occur automatically but, rather, the solution is introduced into the cassette by mechanical manipulation. Such solutions are heat sensitive and therefore must be stored refrigerated. The electrolyte-containing cassette must be heated to 37 degrees celsius prior to use as pH is a function of the temperature of the sample to be measured. After heating, the cassette has a shelf life of approximately one hour. A pH measurement using this method therefore requires proper treatment of the cassette containing the electrolyte before an accurate value can be determined. This can create unwanted delay in emergency situations. Additionally, the apparatus is designed to accommodate only one cassette at a time. Subsequent samples must be loaded into the machine manually and the user must wait a given amount of time before an accurate measurement can be made, thus causing an additional delay in critical circumstances. Further, a reading can only be taken when the cassette is in an upright position. Moreover, the measuring instrument which embodies the subject matter of these patents is large, table-top size, and requires a line source of power in order to perform the heating, measuring and other functions.
A further difficulty is encountered in blood gas measurement systems. Where an electrode system is used in which a measurement circuit is positioned in and out of contact with the electrode system, good, low ohmic contact is difficult to achieve. This is because the electrodes provide signals having a very large source resistance. As such, there is insufficient current flow to produce good low ohmic contact between the electrode system and the measurement circuit. The physics of this is that even with sliding gold contact surfaces there is always a surface layer which will provide some insulation. When working at very high circuit impedances, this layer is sufficient to degrade voltage measurement.